1. Field of the Invention
The present invention relates to a technique for controlling the temperature of a secondary battery formed by combination of a plurality of cells or by a plurality of battery modules in which a plurality of cells are connected in series.
2. Related Art
There has hither to been put forward a technique for heating or warming a battery in order to prevent a decrease in the performance of the battery, which would otherwise arise at low temperature. For instance, proposed in Japanese Patent Laid-Open Publication No. 2001-314039 is a secondary battery input-and-output controller which recharges a battery mounted in a hybrid electric vehicle (hereinafter called a “hybrid vehicle”) through use of regenerated energy, there by increasing the temperature of the battery. By means of this input-and-output controller, the battery at low temperature is heated by the heat of recharging reaction of the battery produced as a result of the recharged state (“State Of Charge” and hereinafter abbreviated as “SOC”) of the battery being subjected to recharging-and-discharging control in a region of low recharging efficiency.
The input-and-output controller described in Japanese Patent Laid-Open Publication No. 2001-314039 heats the battery by the heat of recharging action induced by the energy regenerated during travel. Accordingly, when the vehicle is stationary, the battery cannot be heated. Therefore, when the engine is inactive, there is a chance of the temperature of the battery being lowered by outside air. A decrease in SOC may pose difficulty in cranking (starting the engine), which would otherwise be induced by the power discharged by the battery.
Meanwhile, put forth in Japanese Patent Laid-Open Publication No. 2004-336832 is a battery temperature controller for detecting an SOC of a battery and the temperature of outside air and heating the battery during stoppage of driving operation of a vehicle by use of power from the battery when the SOC of the battery is greater than a predetermined SOC level and the temperature of outside air is lower than a predetermined temperature. The temperature controller described in this patent publication prevents a drop in the temperature of the battery, which would otherwise arise after deactivation of the engine, to thus ensure the ease of activation of the engine.
Incidentally, in many cases, a secondary battery formed by combination of a plurality of single cells or by a plurality of battery modules, each of which is made by connecting a lot of single cells in series, is used for a battery to be mounted in a hybrid vehicle, such as that mentioned above. When the secondary battery structured as mentioned above is heated through use of the controller described in Japanese Patent Laid-Open Publication No. 2001-314039, the controller described in Japanese Patent Laid-Open Publication No. 2004-336832, or another heating means, there may arise a case where variations occur in the plurality of single cells or the plurality of battery modules, which constitute the secondary battery, in terms of a battery temperature and by extension a battery voltage, because of heating characteristics of the heating means or the structure of the secondary battery. Since the single cells or the battery modules are connected in series within the secondary battery such that a desired high voltage is achieved, those variations may sometimes result in deterioration of the secondary battery as a result of appearance of excessively-discharged single cells or battery modules, and as well may cause a drop-off in the performance of the secondary battery.